1. Field of the Invention
This invention relates to a laminated structure useful in a safety glass comprising a glass layer and a plastic layer laminated to each other. The safety glass is used, for example, as a windshield of an automobile and a windowpane of a building.
2. Description of the Prior Art
Attempts have long been made to use a laminated structure comprising a sheet glass, a polyvinylbutyral sheet bonded to the inside surface of the glass sheet and a polyester film bonded further to the inside surface of the polyvinylbutyral sheet as an automobile windshield glass. The advantage of using this laminated structure as a windshield is that in the event of an automobile accident, the object with which the head and face of a driver collide directly is a relatively soft resin layer, and the degree of injury to the head and face is much lower than that in the case of collision with a laminated glass windshield. However, since the outermost layer of this laminated structure is a resin, it is weak to scratching and the application of a strong force will distort the polyvinylbutyral layer and cause a viewing distortion. Attempts are made to eliminate these defects by providing a hard coat on the surface of the polyester film to increase its scratch resistance and adjusting the thicknesses of the polyvinylbutyral sheet, the polyester film and the sheet glass to proper values to provide resistance to distortion.
To ensure sufficient holding of windows of an automobile in a colliding accident, a moderate degree of adhesion strength is required between the window and the body, between the glass and the polyvinylbutyral sheet and between the polyvinylbutyral sheet and the polyester film. It is particularly difficult however to increase the adhesion strength between the polyvinylbutyral sheet and the polyester film, and much research has been conducted in this regard.
Generally, flame treatment, corona discharge treatment, ultraviolet treatment, plasma treatment, alkali treatment and treatment with various chemicals are known as a method of improving the adhesion of a polyester film.
Japanese Laid-Open Patent Publications Nos. 46717/1975, 19314/1978 and 219053/1985 and Japanese Patent Publication No. 37155/1983 propose methods in which the surface of a polyethylene terephthalate film is subjected to a flame treatment. Japanese Laid-Open Patent Publication No. 176831/1987 propose a method of improving adhesion by providing a titanium dioxide between the polyester film and the polyvinylbutyral sheet.
It is known that the surface characteristics of a polyester film are improved by using an amino group-containing organosilicon compound of general formula (I) or a diamine of general formula (II) which are used in this invention [see, for example, Japanese Patent Publication No. 16344/1985, Papers Read in the 30th Meeting of Polymer Research (Kobe), page 34, and Collection of Articles on Polymers, vol. 35, 229 (1978)]. Japanese Patent Publication No. 16344/1985 proposes a method of improving the adhesion between a polyester film and an organopolysiloxane compound layer by using an adhesive composed of a mixture of 3-(2-aminoethylamino)-propyltrimethoxysilane and an epoxy resin. The above-cited Papers Read in the 30th Meeting of Polymer Research (Kobe), page 34 proposes a hexamethylenediamine as a surface-treating agent for polyester films. The above-cited Collection of Articles on Polymers, vol. 35, 229 (1978) proposes hydrazine as a surface-treating agent for polyester films.
However, with the flame treatment, the corona discharge treatment, the use of a surface-treating agent composed only of hexamethylenediamine and the use of a surface-treating agent composed only of hydrazine in the prior art, no sufficient results have been obtained with regard- to the adhesion strength.
With the use of the mixture of 3-(2-aminoethylamino)-propyltrimethoxysilane and an epoxy resin and the alkali treatment, it is necessary to perform the curing reaction or the treatment for as long as several tens of minutes for obtaining sufficient adhesion. Accordingly, these methods have very low productivity, and incur a high cost.
Moreover, the flame treatment, corona discharge treatment, plasma treatment and alkali treatment undesirably result in a reduction in film strength.